1. Field of the Invention
This invention relates to storage stable polyorganosiloxane compositions. More particularly, this invention relates to one part polyorganosiloxane compositions curable by a platinum catalyzed hydrosilation reaction. The compositions contain a catalyst inhibitor that imparts long term storage stability at 50.degree. C., yet allows the compositions to cure rapidly to a gel at temperatures of about 135.degree. C. and above.
2. Description of the Prior Art
Polyorganosiloxane compositions which cure by means of a platinum catalyzed hydrosilation reaction to form silicone gels are disclosed, for example, in U.S. Pat. Nos. 3,020,260, issued to Nelson on Feb. 6, 1962, and in 4,374,967, issued to Brown, Lee and Maxson on Feb. 22, 1983. The gels described by Nelson are obtained by reacting an organosiloxane containing two silicon-bonded hydrogen atoms per molecule with a vinyl-containing copolymer wherein the repeating units are of the formulae RViSiO, R.sub.2 SiO, and CH.sub.3 R.sub.2 SiO.sub.0.5, where R is methyl or phenyl and Vi represents vinyl. The vinyl-containing copolymers disclosed by Brown et al. consist essentially of (CH.sub.3).sub.2 SiO, (CH.sub.3).sub.3 SiO.sub.0.5 and (CH.sub.3).sub.2 ViSiO.sub.0.5, where Vi again represents vinyl, and are reacted with an organohydrogensiloxane having more than one silicon-bonded hydrogen atom per molecule. Brown et al. teach that known inhibitors for platinum catalyzed hydrosilation reactions can be added to increase the working time of catalyzed compositions by decreasing the rate at which these compositions cure at temperatures above about 100.degree. C. While known inhibitors will partially suppress the reactivity of catalyzed compositions containing both vinyl radicals and silicon-bonded hydrogen atoms, these compositions cannot be stored for extended time periods under ambient conditions without either undergoing a substantial increase in viscosity or curing to form a solid gel. A commercially useful one-part composition should not vary more than about 10% in viscosity during the period of several weeks that can elapse between the time the composition is prepared and the time at which it is desired to cure the composition. During this period the compositions are stored in locations where the ambient temperature may reach 40.degree. C.
Until now, it has not been feasible to prepare one-part compositions of the type described in the aforementioned Nelson and Brown et al. patents that meet the foregoing stability requirements for commercially useful materials. Inhibitors that have been disclosed as useful catalyst inhibitors for the compositions taught in the aforementioned Brown et al. patent include the aromatic hetrocyclic nitrogen compounds, pyridazine, pyrazine, quinoline, 2,2'-biquinoline, bipyridine, naphthyridine, quinaldine, dialkyl formamides, thioamides, alkylthioureas and ethylene thiourea described in U.S. Pat. No. 3,188,299, issued June 8, 1965, the organophosphorus compounds as described in U.S. Pat. No. 3,188,300, issued June 8, 1965, benzotriazole as described in U.S. Pat. No. 3,192,181, issued June 29, 1965, the nitrile compounds as described in U.S. Pat. No. 3,344,111, issued Sept. 26, 1967, the halocarbons described in U.S. Pat. No. 3,383,356, issued May 14, 1968, the acetylenic compounds described in U.S. Pat. No. 3,445,420, issued May 20, 1969, the vinyl silazanes described in U.S. Pat. No. 3,453,233, issued July 1, 1969, the sulfoxide compounds described in U.S. Pat. No. 3,453,234, issued July 1, 1969, the stannous salts, mercuric salts and other salts described in U.S. Pat. No. 3,532,649, issued Oct. 6, 1970 and the cyclic polymethylvinylsiloxanes discussed in U.S. Pat. No. 3,923,705, issued Dec. 2, 1975.
The use of ammonia, amines, and other nitrogen-containing compounds as inhibitors for the platinum catalyzed reaction of silicon-bonded hydrogen atoms with silicon-bonded hydroxyl groups is taught in U.S. Pat. No. 3,867,343, issued to Garden on Feb. 18, 1975. U.S. Pat. No. 4,281,093, issued to Garden on July 28, 1981, discloses using these inhibitors for the reaction of SiH-containing polyorganosiloxanes with polyorganosiloxanes containing silicon-bonded hydroxyl, vinyl or allyl groups. The catalysts for the reaction are organometallic complexes of platinum or rhodium.
The longest gel time, i.e., time required for gelling of the reaction mixture at 25.degree. C., reported in U.S. Pat. No. 3,867,343 is 2409 minutes, equivalent to 40.15 hours, and is obtained using alpha-dipyridyl. This value does not take into account the increase in viscosity that typically occurs prior to complete gelation. The viscosity increase could render the composition unsuitable for commercial use as a one-part composition considerably prior to the time gelation occurred. N,N,N',N'-tetramethylethylenediamine was considerably less effective as an inhibitor than alpha-dipyridyl, imparting a gel time of 1533 minutes (25.5 hours). The data in U.S. Pat. No. 3,867,343 would not encourage the evaluation of amines and other nitrogen compounds disclosed therein as catalyst inhibitors for one-part platinum catalyzed polyorganosiloxane compositions that must remain stable for several weeks at 50.degree. C. prior to being cured, and which cure to a gel in less than 60 minutes at 135.degree. C. Preferred compositions cure in less than 20 minutes and are stable for several months at 50.degree. C.